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Biomedical subjects

R D Voyksner

Publications and source records attributed to R D Voyksner.

At least 19 recordsLinked to original sources
PubMed2005

In vitro metabolism of an orally active O-methyl amidoxime prodrug for the treatment of CNS trypanosomiasis.

A new aza-analogue of furamidine, 6-[5-(4-amidinophenyl)-furan-2-yl]nicotinamidine (DB820), has potent in vitro antitrypanosomal activity; however, it suffers from poor oral activity because of its positively charged amidine groups. The dimethoxyamidine prodrug of DB820, N-methoxy-6-{5-[4-(N-methoxyamidino)phenyl]-furan-2-yl}-nicotinamidine (DB844), has potent oral activity in mouse models of both early-stage and CNS African trypanosomiasis. Metabolism of DB844 in human liver microsomes (HLM) was investigated using liquid chromatography-mass spectrometry (LC-MS/MS). The metabolism of DB844 in HLM was NADPH-dependent and resulted in the production of eight metabolites over a 90?min incubation. O-Demethylation and N-dehydroxylation reactions resulted in the metabolic conversion of DB844 to its active DB820 metabolite. Chromatographic conditions used for LC-MS analysis allowed for the separation and identification of all metabolites including positional isomers. Demethylation of either the phenyl or pyridine side of DB844 (DB844 m/z 366.2) resulted in the production of two metabolites (M1A, M1B), each with a molecular ion of m/z of 352.3 and MS(2) fragments of 288.1, 305.2, 321.2 and 335.2. However, the intensities of the MS(2) fragments were different among the two isomeric metabolites, and comparison to an authentic standard allowed for the structural determination of each metabolite. The isomeric metabolites M2A and M2B, resulting from amidoxime reductions of M1A and M1B, were also chromatographically separated and had distinguishable MS(2) profiles that allowed for their structural assignments when compared to an authentic standard. The di-amidoxime product resulting from O-demethylation of either side of DB844 was also identified as an abundant metabolite during microsomal incubations. The active antitrypanosomal metabolite, DB820, was the last metabolite to be formed and thus provides evidence that DB844 may effectively be metabolized to its active metabolite in vivo.

Administration, Oral↗
PubMed2000

Analysis of pesticides by LC-electrospray-MS with postcolumn removal of nonvolatile buffers.

Liquid chromatographic (LC) separations for pesticides and many other compounds make use of nonvolatile buffers in the mobile phase. The coupling of LC with mass spectrometry (MS) does not allow the use of nonvolatile buffers. Substitution with volatile buffers is possible, but changes in chromatographic retention and resolution can result even if pH is held constant. The postcolumn removal of nonvolatile buffers using a commercially available ion suppressor is evaluated for the analysis of carbamate pesticides. The suppressor efficiently removes phosphate anions from an LC mobile phase. Most compounds show an increased signal by factors of 2-7 after postcolumn phosphate removal. The suppressor has little effect on the chromatographic parameters of some compounds, while serious negative effects are noted for others. Some compounds will give poor results due to adsorption or retention by the suppressor. The results indicate that such a device may be useful for the LC-MS analysis of some pesticides using nonvolatile buffers.

Carbofuran↗
PubMed2000

Evaluation of electrospray transport CID for the generation of searchable libraries.

This paper reports the preliminary investigation of performance-based standard conditions that have been developed for electrospray ionization mass spectrometry. Using performance-based standard criteria, reproducible spectra can be obtained by CID in the electrospray-transport region and searched using a database created using the same performance criteria. To generate library-searchable mass spectra, the instrument was tuned to standard conditions that correspond to low, mid, and high fragmentation energies. The instrument was tuned using ion ratios relative to a given peak in a tune compound for each energy level. The library was evaluated using a set of 22 benzodiazepines. The CID libraries were found to be reproducible, both on the same instrument and on different instruments of the same type. Also, the libraries were found to be independent of flow rate and solvent system. The library was expanded to include 16 sulfonylurea herbicides and tested using spiked water samples. Performance of the library was tested over a concentration range of 2 orders of magnitude using sulfonylurea standards.

Benzodiazepines↗
PubMed1999

Investigating the use of an octupole ion guide for ion storage and high-pass mass filtering to improve the quantitative performance of electrospray ion trap mass spectrometry.

An octupole radio frequency (rf) ion guide was evaluated for storage and filtering of ions generated by electrospray prior to introduction into an ion trap mass spectrometer. The control of the rf potential on the ion guide enabled its operation as a high-pass mass filter, removing low-mass chemical noise that would normally fill the ion trap and result in reduced sensitivity, mass resolution, and dynamic range. Also, the ion guide can serve to store the high-pass filtered ions during the ion trap mass analysis, enabling nearly 100% duty cycle and reduction of the cycle time by a factor of two. The linearity, precision, and detection limits of the liquid chromatography (LC) electrospray - ion guide-ion trap MS/MS system were evaluated for the determination of ceftiofur in milk. A linear calibration (linear correlation coefficient of 0.986) from 2-200 ppb was obtained with a relative standard deviation for replicate analysis of less than 8%. A quantitation detection limit of 100 pg of ceftiofur on-column (2 ppb) was achieved from a milk extract. This detection limit is nearly a factor of 10 lower compared with the determination on the same electrospray ion trap system not using a rf ion guide for high-pass mass filtering and ion storage.

Animals↗
PubMed1998

Quantitative determination of ceftiofur in milk by liquid chromatography-electrospray mass spectrometry.

A liquid chromatography-electrospray mass spectrometry (LC-ES-MS) was developed for the quantitation of ceftiofur in milk at the 50 ppb tolerance level set by the US Food and Drug Administration (FDA) for the drug. The method used ultrafiltration as a simple and rapid means to prepare the sample for analysis. A 100 microliters volume of ultrafiltrate containing ceftiofur was concentrated on-column for LC-MS analysis. The LC separation was accomplished using an acetonitrile gradient with the ion-pair reagent heptafluorobutyric acid (HFBA). Propionic acid was added after the LC column to minimize electrospray signal suppression, enhancing the response for ceftiofur by a factor of 10. The transmission ions from the electrospray interface to the MS was enhanced by a factor of 7 by using a Rf ion guide. The development method could detect ceftiofur to 10 ppb and quantitate the antibiotic from 25-200 ppb (linear correlation coefficient of 0.993). The analysis indicated that bovine milk collected 32 h after dosing with ceftiofur was above the FDA tolerance of 50 ppb, while milk collected 48 h after dosing was found to contain 24-31 ppb of ceftiofur.

Animals↗
PubMed1994

Determination of beta-lactam residues in milk using perfusive-particle liquid chromatography combined with ultrasonic nebulization electrospray mass spectrometry.

The introduced electrospray (ESP) technique combined with quadrupole mass spectrometry (MS) was applied for the trace residue detection (10 ppb) of commonly administered beta-lactam antibiotics in bovine milk. Because of the widespread use of penicillin G, ampicillin, amoxicillin, cephapirin, cloxacillin, and ceftiofur in veterinary medicine, these six popular drugs were chosen to develop a fast and reliable microcolumn liquid chromatographic (micro-LC) separation method for residue-containing milk extracts. Furthermore, the analytes were selected to test the chromatographic behavior of a novel stationary phase, a perfusive-particle column packed with derivatized porous polystyrene divinylbenzene. The effects of mobile phase additives on separation and ESP ionization efficiency were investigated. The ionization of the eluted analytes took place either in a conventional ESP interface with a capillary shield allowing flow rates up to 40 microL/min or in the latest model of the ultrasonic nebulization ESP interface. The perfusive-particle column, although not optimized for the separation of small molecules, exhibited sufficient resolution of the antibiotics for its routine usage, especially when considering the remarkable time-saving advantage compared with ordinary reversed phase micro-LC columns. The application of the novel ultrasonic nebulization interface lowered detection limits and improved the stability of the signals over a much wider flow rate range than possible for ionization with the ESP system equipped with the conventional needle.

Animals↗
PubMed1994

Simultaneous determination of enrofloxacin and its primary metabolite ciprofloxacin in bovine milk and plasma by ion-pairing liquid chromatography.

A simple and sensitive high-performance liquid chromatographic method has been developed for the simultaneous determination of enrofloxacin and ciprofloxacin in bovine milk and plasma. Sample preparation consisted of mixing equal volumes of milk or plasma with acetonitrile-0.1 M sodium hydroxide (1:1, v/v), followed by ultrafiltration through 3000 Da molecular mass cut-off filters. Separation of these two fluoroquinolones in milk or plasma ultrafiltrate was accomplished by ion-pairing liquid chromatography using a reversed-phase analytical column eluted with acetonitrile-methanol-water. Ultraviolet absorbance of the column effluent was monitored over the 230-350 nm range with a photodiode-array detector (lambda max 278 nm). Recoveries of enrofloxacin from bovine milk and plasma were 92-107% and 80-84%, respectively. Recoveries of ciprofloxacin from bovine milk and plasma were 92-105% and 73-75%, respectively. The limit of detection for the two compounds was 5 ng/ml. Enrofloxacin was administered intravenously to a lactating cow at a dose of 2.5 mg/kg. Enrofloxacin was detected in milk within 15 min after injection and the metabolite ciprofloxacin rapidly appeared in plasma and milk. Both enrofloxacin and ciprofloxacin were below the limit of detection (5 ng/ml) by 48 h after drug administration.

Animals↗
PubMed1994

Analysis of neuropeptides by perfusion liquid chromatography/electrospray ion-trap mass spectrometry.

Perfusion high-performance liquid chromatography (HPLC) combined with electrospray ion trap mass spectrometry (ITMS) was evaluated for the determination of neuropeptides in plasma. Perfusion HPLC offers the capability of resolving neuropeptides spiked into plasma in 5 min compared to the 30-60 min separations performed on packed capillary C18 columns. Electrospray combined with the ITMS provides the ability to ionize these neuropeptides and mass analyze them with high sensitivity and specificity. Sub-picomole quantities of neuropeptides injected on-column could be specifically detected in a plasma matrix. The electrospray-ITMS mass spectrum of each neuropeptide showed multiply charged ions which could be used to determine or confirm their molecular weights.

Chromatography, High Pressure Liquid↗
PubMed1993

Determination of penicillin G, ampicillin, amoxicillin, cloxacillin and cephapirin by high-performance liquid chromatography-electrospray mass spectrometry.

This report contributes to a preliminary investigation of high-performance liquid chromatographic (HPLC)-mass spectrometric (MS) methods for confirming beta-lactam antibiotic residues in bovine milk. Initial work for each antibiotic evaluated the collisional activated dissociation (CAD) spectra that could be generated between the capillary and skimmer in the electrospray (ESP) interface. The drugs show various characteristic fragmentation, mostly within the beta-lactam ring and the amide group. Response for a particular compound in a given solvent can vary drastically. Usually, the more organic component in the solvent, the higher the ESP response. In many cases use of acetonitrile also results in slightly better ion currents than for methanol when comparing equal percentages of either organic solvent in water. The ESP response of most of the tested antibiotics can be enhanced by the addition of formic acid or acetic acid to the mobile phase methanol-water (1:1). In general, the negative ion spectra are lower in intensity, exhibiting an [M-H]- ion and producing less fragmentation at higher CAD voltages as compared to positive ion spectra. An isocratic reversed-phase HPLC method for the separation of a mixture of five common beta-lactam antibiotics was developed using acetic acid as a mobile phase additive and optimized for detection with a new ESP HPLC-MS interface. A post-column split ratio of 70:1 for the eluent from a 150 x 2 mm I.D. column was chosen to provide the required lower flow-rate (approximately 4 microliters/min). The limit of detection for the simultaneous determination of these antibiotics was estimated to be 100 ppb. Electrospray HPLC-MS could be used to confirm these antibiotics for quantities down to about 100 pg entering the mass spectrometer. Multiresidue analysis with microbore HPLC-ESP-MS has the advantage that no post-column splitting of the eluent is required and all of the analyte (on-column injected) will be transferred into the ESP interface. Preliminary work showed good mass spectrometric sensitivity down to the level of regulatory interest, but chromatographic separation efficiency must be improved.

Amoxicillin↗
PubMed1993

Determination of ceftiofur and its metabolite desfuroylceftiofur in bovine serum and milk by ion-paired liquid chromatography.

A simple and sensitive liquid chromatographic method has been developed for the simultaneous determination of ceftiofur and its metabolite desfuroylceftiofur in bovine serum and milk. The method involved an ultrafiltration of diluted serum/milk with an equal volume of 50% acetonitrile through a 10,000 dalton molecular mass cut-off filter. Separation of ceftiofur and desfuroylceftiofur from the other serum/milk components was performed by ion-paired (octane and dodecanesulfonate) liquid chromatography using a reversed-phase column eluted with acetonitrile-water solution. The ultraviolet-visible absorbance of the column effluent was monitored in 200-350 nm range of a photodiode-array detector or at lambda max 289.6 nm for ceftiofur, lambda max 265.8 nm for desfuroylceftiofur and lambda max 271.4 nm for dimer of desfuroylceftiofur. Recoveries of ceftiofur from bovine milk spiked with 1 and 10 micrograms/ml were 95.9 and 97.0% with coefficients of variation of 3.69 and 2.51%, respectively. Recovery of ceftiofur from bovine serum spiked with 10 micrograms/ml was 90.4% with a coefficient of variation of 5.29%. A correlation coefficient of 0.9992 occurred with ceftiofur in aqueous solutions (n = 5, in duplicates). The limit of detection was estimated to be approximately 50 ppb (ng/ml). Additionally, this paper documents the presence of a ceftiofur metabolite in bovine serum under in vitro and in vivo conditions. The metabolite was identified as desfuroylceftiofur together with its dimer 3,3'-desfuroylceftiofur disulfide by thermospray liquid chromatography-mass spectrometry.

Animals↗
PubMed1991

Development of analytical methods for some penicillins in bovine milk by ion-paired chromatography and confirmation by thermospray mass spectrometry.

Analytical methods for the determination of cloxacillin, ampicillin/hetacillin, and amoxicillin in bovine milk were developed. The methods involved ultrafiltration of milk diluted with methanol, acetonitrile, and water on a 10,000-dalton cut-off filter. Separation of penicillins from other milk components was accomplished by ion-paired chromatography using a microbore column. The penicillins were detected using ultraviolet photodiode array (UV-PDA) detection and confirmed by thermospray liquid chromatography-mass spectrometry (LC-MS). The thermospray spectra of these compounds exhibited [M + H]+ and [M + Na]+ ions along with several fragment ions. The limits of detection for these antibiotics were estimated to be 50 to 100 ppb for LC with UV-PDA detection and 100-200 ppb for thermospray LC-MS detection.

Animals↗
PubMed1991

Investigation of collisional-activation decomposition process and spectra in the transport region of an electrospray single-quadrupole mass spectrometer.

The use of collisional-activation dissociation (CAD) in the electrospray transport region was evaluated for generating structural information on several pesticides and antibiotics. The collision energy used to generate the CAD spectra could be varied easily by changing the capillary/skimmer potential difference, imparting from 0 eV to above 16 eV internal energy to the near thermal ions generated by electrospray. The internal energy distribution for low-energy collisions (capillary/skimmer potential difference of 20 V) closely matches the curves generated by a triple-quadrupole mass spectrometer. Furthermore, the CAD spectra for selected compounds generated by electrospray in the transport region at a capillary/skimmer potential difference of 30-50 V closely resembled those obtained from the [M + H]+ ion by a triple quadrupole using 30 eV collision energy. The CAD of ions in the transport region resulted in 70% to 80% daughter-ion yields and minimal loss in overall ion current compared to the ion current for protonated or cationized parent molecules. The major daughter ions for 10 pg of Aldicarb and penicillin G could be detected (signal-to-noise ratio greater than 5) under full-scan CAD conditions.

Anti-Bacterial Agents↗
PubMed1991

Development of an analytical method for cephapirin and its metabolite in bovine milk and serum by liquid chromatography with UV-VIS detection and confirmation by thermospray mass spectometry.

Metabolites of the cephapirin beta-lactam antibiotic have not previously been reported in bovine milk. The principal metabolite was tentatively identified as desacetylcephapirin by liquid chromatography with UV-VIS photodiode array (LC/UV-VIS PDA), and liquid-chromatography-mass-spectrometric (LC-MS) detection. Synthetic desacetylcephapirin was prepared by incubation of cephapirin in bovine milk and serum at 37 degrees C. Also, a method for determining cephapirin in bovine milk and serum was developed. The detection limits for cephapirin and desacetylcephapirin were estimated to be 10 and 50 micrograms/kg, respectively, for LC/UV-VIS PDA, and 100 and 500 micrograms/kg for LC-MS.

Animals↗
PubMed1991

Effect of SRI 63-675 on hemodynamics and blood PAF levels during porcine endotoxemia.

We evaluated the effect of SRI 63-675, a specific platelet-activating factor (PAF) receptor antagonist, on hemodynamics and PAF biosynthesis during 4 h of porcine endotoxemia. Hexadecyl PAF was extracted from blood, purified by normal-phase and reverse-phase high-performance liquid chromatography (RP-HPLC), and quantitated by stable isotope dilution and thermospray mass spectrometry. Infusion of either saline or SRI 63-675 alone caused no change in hexadecyl PAF concentrations. In contrast, endotoxin increased blood hexadecyl PAF concentrations from 1.5 +/- 0.1 ng/ml at 0 h (baseline) to a peak value of 8.3 +/- 1.9 ng/ml at 0.5 h of endotoxemia (P less than 0.05). Blood PAF levels gradually declined toward the baseline value after 0.5 h of endotoxemia. Because endotoxin did not modify plasma acetylhydrolase activity ex vivo, the increased hexadecyl PAF levels were probably secondary to increased PAF biosynthesis and not decreased biodegradation. Bioassay of RP-HPLC fractions that were derived from endotoxemic blood and that eluted at a retention time consistent with [3H]alkyl PAF caused aggregation of washed rabbit platelets that was inhibited by SRI 63-675. The PAF receptor antagonist blocked the 0.5 h endotoxin-induced increase in blood hexadecyl PAF concentration concomitant with blockade of thrombocytopenia. The endotoxin-induced pulmonary hypertension, decreased cardiac index, and increases in pulmonary vascular resistance and alveolar-arterial O2 gradient were attenuated by SRI 63-675. The data suggest that PAF-stimulated PAF biosynthesis may substantially contribute to blood hexadecyl PAF levels and cardiopulmonary dysfunction during the initial phase of endotoxemia.

Animals↗
PubMed1990

Application of countercurrent chromatography/thermospray mass spectrometry for the identification of bioactive lignans from plant natural products.

The versatility and resolving power of countercurrent chromatography (CCC) has been demonstrated with a newly developed analytical high-speed planet centrifuge system. Interfacing countercurrent chromatography with mass spectrometry (MS) provides a new analytical methodology that integrates the advantages of countercurrent chromatography with the low detection limit and identification capability of mass spectrometry. In this paper the capability of thermospray CCC/MS is demonstrated in identifying and validating the bioactive and structurally known lignans from a crude extract of Schisandra rubriflora Rhed et Wils, a traditional Chinese herbal medicine for treatment of hepatitis.

Chromatography↗
PubMed1990

Optimization and application of particle beam high-performance liquid chromatography/mass spectrometry to compounds of pharmaceutical interest.

Particle beam high-performance liquid chromatography/mass spectrometry (HPLC/MS) parameters were optimized for the sensitive analysis of several drugs in agricultural products such as milk and tissue. Sensitivity of the particle beam interface was greatest for solvents with low heat capacities (methanol greater than acetonitrile greater than isopropanol greater than water). Furthermore, optimal sensitivity was obtained at low solvent flow rates (about 0.4-0.6 ml min-1). Parameters such as desolvation temperature, helium flow rate to the nebulizer and nebulizer position resulted in minimal change in sensitivity. The source temperature was optimized to obtain suitable vaporization with minimal thermal degradation (200-300 degrees C). The determination of a variety of compounds (including beta-lactams, cephapirin, tetracyclines, methylene blue, furosemide, spectinomycin, cytidine, 2-chloro-4-nitrobenzamide and thiamine) was possible using the particle beam interface. Under full-scan conditions, detection limits were in the 100 ng range for most drugs. With selected ion monitoring, particle beam HPLC/MS was demonstrated for the analysis of p.p.m. levels of these drugs in milk and tissue extracts. Precision of the particle beam analysis was usually better than 15% RSD. For the same compounds, HPLC/MS with a thermospray interface often resulted in less structural information (single ion spectra) then obtained by particle beam with thermospray detection limits varying from 10 ng to 1 microgram.

Animals↗
PubMed1990

Optimization of immobilized enzyme hydrolysis combined with high-performance liquid chromatography/thermospray mass spectrometry for the determination of neuropeptides.

Peptidases, including chymotrypsin, thermolysin, trypsin, V8 protease, and carboxypeptidases A, B, and Y, were immobilized for use in conjunction with HPLC/thermospray MS for the analysis of neuropeptides. The optimal operating conditions for each immobilized enzyme bioreactor were determined. Optimal hydrolysis usually occurred at the highest percentage of aqueous solution in the mobile phase at pH 7-8 and 40-50 degrees C. Often post-HPLC column addition of aqueous solutions before the bioreactor could improve activity and thermospray sensitivity without changing the HPLC separation. Enzymatic hydrolysis requirements were compatible under conditions for HPLC separation and thermospray MS detection of the selected neuropeptides. Synthetic alpha-, beta-, and gamma-endorphins were the primary neuropeptides used to evaluate on-line immobilized enzyme bioreactor/MS. HPLC followed by peptidase hydrolysis produced characteristic hydrolysis products for confirming the peptides' identity using thermospray MS detection. Furthermore, the peptide formed from enzymatic hydrolysis resulted in a MS ion current 10-40 times higher than that of the [M + 2H]2+ ion for unhydrolyzed beta-endorphin. The increased sensitivity achieved for detecting the hydrolysis products permits detection and quantitation of synthetic peptides down to 800 fmol.

Chromatography, High Pressure Liquid↗